(1) We intend to measure binding of TN-I to actin filaments free of tropomyosin and determine the inhibition as a function of bound rather than added TN-I. (2) We shall compare the maximal amount of TN-I bound in the presence and in the absence of tropomyosin. (3) In order to see whether the actin binding site of TN-I is still available when TN-I is part of the complete troponin molecule we shall measure the binding of troponin to actin filaments free of tropomyosin. We shall make these measurements in the presence and absence of calcium. (4) We plan to determine the effects of phosphorylation of TN-I and TN-T on the properties of the calcium switch of troponin from cardiac as well as skeletal muscle.